RF-only multipole structures are widely used in mass spectrometers as ion guides and/or collision cells. Generally described, RF-only multipoles consist of four or more elongated rods that bound an interior region through which ions are transmitted. The ions enter and exit the multipole rod set axially. A radio-frequency (RF) voltage is applied to opposed rod pairs to generate an RF field which confines the ions radially and prevents ion loss arising from collision with the rods. RF-only multipoles are operationally distinguishable from standard quadrupole mass filters, which utilize a DC electric field component in the radial plane to enable separation of ions according to mass-to-charge (m/z) ratio; as the name denotes, RF-only multipoles omit the DC field component in the radial plane and thus allow passage of ions having differing m/z ratios.
In many mass spectrometers, the ion source (such as an electrospray ionization (ESI) source, an atmospheric pressure chemical ionization (APCI) sources, as well as certain types of matrix-assisted laser desorption ionization (MALDI) sources) operates at a significantly higher pressure relative to the pressure in the mass analyzer region. Due to collisional damping effects (which reduce the kinetic energy of ions within the multipole) it may be desirable or necessary to provide an axial DC field in an RF-only multipole located in a high-pressure or intermediate-pressure region to assist in propelling the ions along the longitudinal axis of the multipole. Generation of the axial DC field is commonly achieved by using (i) segmented RF-only multipoles with variable DC offset voltage between segments; (ii) tilted or shaped appropriately auxiliary metal rods positioned in gaps between RF rods; or, (iii) a set of supplemental auxiliary rods (metal segments or isolator covered with resistive material), located between the main RF rods and being arranged substantially parallel thereto. In the last case, an axial DC potential gradient is created by applying a first voltage to corresponding first ends of the auxiliary rods and a second voltage to corresponding second (opposite) rod ends. The use of auxiliary rods and related techniques for generating an axial DC field in RF-only multipoles is disclosed in, for example U.S. Pat. No. 6,111,250 by Thomson et al., entitled “Quadrupole with Axial DC Field.”
The implementation of auxiliary rods in RF-only multipoles is often problematic and may complicate the operation and/or compromise the performance of mass spectrometers. A notable operationally significant problem is that the DC potential in the radial plane orthogonal to the major longitudinal axis of the multipole may vary significantly with angular and radial position, being dependent upon the geometry of both rod sets and the differences in DC voltages applied. Poor homogeneity of DC potential may adversely affect ion transmission efficiency, especially when large excursion of ion trajectories from the major longitudinal axis occur. Additionally, the presence of the auxiliary rod set may interfere with the optical pathway of the laser beam used to desorb and ionize the sample. In view of these problems and disadvantages, there is a need in the art for an improved technique for providing an axial DC field in an RF-only multipole.